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The oil pressure relief valve is located inside the engine, screwed into the oil pan. The purpose of the valve is bleed off any pressure that is in excess of the official specifications (typcially 3.8 - 4.2 kg/cm2 or 54 - 60 PSI). When the oil pressure is higher than this amount, this valve opens to bring it down to the specified limits.
Unfortunately, these valves are notorious for leaking off pressure long before the factory specs. The problem is that the valve does not make a good seal against its seat. Oil then leaks past before the valve opens. This is not a great situation. Every time I rebuild an engine, I check this valve and, if needed, I clean up the mating surface between the valve and its seat.
To clean up the mating surface, I apply valve grinding paste to the mating surface. Then, I rotate the valve using a flat tipped driver attachment fit to my cordless drill.
In order to know how the valve is performing, you must have a means of testing the valve. I've created an oil pressure relief valve test aparatus for Loopframes. Paul Greenberg created a test aparatus for the Tonti framed Guzzis. In Paul's own words and photos:
For completeness, a picture of the adapter I made to test the newer style relief valves. The base thread is 14 mm × 1.5 mm, and I threaded the other end of the brass tube for 1⁄8 inch pipe for convenience.
Oil pressure relief valve test aparatus for Tonti frames.
